1. Technical Field
The present system and method relate generally to the reduction of pollutants from emissions released by automotive engines, and more particularly to the optimization of reduction of pollutants in exhaust emissions where parameters for operation of the engine and an after-treatment device are adjusted according to the cost of operation.
2. Description of the Related Art
Due to very high thermal efficiencies, the diesel engine offers good fuel economy and low emissions of hydrocarbons (HC) and carbon monoxide (CO). Despite these benefits, more efficient operation of diesel engines results in higher emissions of nitrogen oxides, i.e., NO or NO2, known collectively as NOx. In diesel engines, the air-fuel mixture in the combustion chamber is compressed to an extremely high pressure, causing the temperature to increase until the fuel's auto-ignition temperature is reached. The air-to-fuel ratio for diesel engines is much leaner (more air per unit of fuel) than for gasoline engines, and the larger amount of air promotes more complete fuel combustion and better fuel efficiency. As a result, emissions of hydrocarbons and carbon monoxide are lower for diesel engines than for gasoline engines. However, with the higher pressures and temperatures in the diesel engine, NOx emissions tend to be higher, because the high temperatures cause the oxygen and nitrogen in the intake air to combine as nitrogen oxides.
NOx emissions from diesel engines pose a number of health and environmental concerns. Once in the atmosphere, NOx reacts with volatile organic compounds or hydrocarbons in the presence of sunlight to form ozone, leading to smog formation. Ozone is corrosive and contributes to many pulmonary function problems, for instance.
Due to the damaging effects, governmental agencies have imposed increasingly stringent restrictions for NOx emissions. Two mechanisms can be implemented to comply with emission control regulations: manipulation of engine operating characteristics and implementation of after-treatment control technologies.
In general, manipulating engine operating characteristics to lower NOx emissions can be accomplished by lowering the intake temperature, reducing power output, retarding the injector timing, reducing the coolant temperature, and/or reducing the combustion temperature.
For example, cooled exhaust gas recirculation (EGR) is well known and is the method that most engine manufacturers are using to meet environmental regulations. When an engine uses EGR, a percentage of the exhaust gases are drawn or forced back into the intake and mixed with the fresh air and fuel that enters the combustion chamber. The air from the EGR lowers the peak flame temperatures inside the combustion chamber. Intake air dilution causes most of the NOx reduction by decreasing the O2 concentration in the combustion process. To a smaller degree, the air also absorbs some heat, further cooling the process.
In addition to EGR, designing electronic controls and improving fuel injectors to deliver fuel at the best combination of injection pressure, injection timing, and spray location allows the engine to burn fuel efficiently without causing temperature spikes that increase NOx emissions. For instance, controlling the timing of the start of injection of fuel into the cylinders impacts emissions as well as fuel efficiency. Advancing the start of injection, so that fuel is injected when the piston is further away from top dead center (TDC), results in higher in-cylinder pressure and higher fuel efficiency, but also results in higher NOx emissions. On the other hand, retarding the start of injection delays combustion, but lowers NOx emissions. Due to the delayed injection, most of the fuel is combusted at lower peak temperatures, reducing NOx formation.
Engine control modules (ECM's), also known as engine control units (ECU's), control the engine and other functions in the vehicle. ECM's can receive a variety of inputs to determine how to control the engine and other functions in the vehicle. With regard to NOx reduction, the ECM can manipulate the parameters of engine operation, such as EGR and fuel injection.
Reducing NOx by manipulating engine operation generally reduces fuel efficiency. Moreover, the mere manipulation of engine operation may not sufficiently reduce the amount of NOx to mandated levels. As a result, after-treatment systems also need to be implemented. In general, catalysts are used to treat engine exhaust and convert pollutants, such as carbon monoxide, hydrocarbons, as well as NOx, into harmless gases. In particular, to reduce NOx emissions, diesel engines on automotive vehicles can employ a catalytic system known as a urea-based Selective Catalytic Reduction (SCR) system. Fuel efficiency benefits of 3 to 10% can result from using SCR systems to reduce NOx rather than manipulating engine operation for NOx reduction which negatively impacts fuel efficiency. Urea-based SCR systems can be viewed according to four major subsystems: the injection subsystem that introduces urea into the exhaust stream, the urea vaporization and mixing subsystem, the exhaust pipe subsystem, and the catalyst subsystem. Several SCR catalysts are available for diesel engines, including platinum, vanadium, and zeolite.
ECM's can also control the operating parameters of catalytic converters, such as urea injection in an SCR system. For instance, the ECM can meter urea solution into the exhaust stream at a rate calculated from an algorithm which estimates the amount of NOx present in the exhaust stream as a function of engine operating conditions, e.g. vehicle speed and load.
The diesel vehicle must carry a supply of urea solution for the SCR system, typically 32.5% urea in water by weight. The urea solution is pumped from the tank and sprayed through an atomizing nozzle into the exhaust gas stream. Complete mixing of urea with exhaust gases and uniform flow distribution are critical in achieving high NOx reductions.
Urea-based SCR systems use gaseous ammonia to reduce NOx. During thermolysis, the heat of the gas breaks the urea (CO(NH2)2) down into ammonia (NH3) and hydrocyanic acid (HCNO). The ammonia and the HCNO then meet the SCR catalyst where the ammonia is absorbed and the HCNO is further decomposed through hydrolysis into ammonia. When the ammonia is absorbed, it reacts with the NOx to produce water, oxygen gas (O2), and nitrogen gas (N2). The amount of ammonia injected into the exhaust stream is a critical operating parameter. The required ratio of ammonia to NOx is typically stoichiometric. The ratio of ammonia to NOx must be maintained to assure high levels of NOx reduction. However, the SCR system can never achieve 100% NOx reduction due to imperfect mixing, etc. In addition, too much ammonia cannot be present. Ammonia that is not reacted will slip through the SCR catalyst bed and exhaust to the atmosphere. Ammonia slip is a regulated parameter which may not exceed a fixed concentration in the SCR exhaust.
Urea-based SCR catalysts can be very effective in reducing the amount of NOx released into the air and meeting stringent emissions requirements. However, the use of urea-based SCR is met with infrastructure and distribution considerations. As described above, diesel vehicles employing urea-based SCR generally carry a supply of aqueous solution of urea, so a urea distribution system is required to allow vehicles to replenish their supplies of urea. The United States currently has no automotive urea infrastructure. The cost of urea is likely to be volatile in the U.S. even as the first pieces of an infrastructure are put in place, because the development of the urea infrastructure is likely to be slow.
In areas, such as Europe, where the price of diesel fuel is generally much higher than the expected price of urea, the SCR system can use as much urea as necessary to reduce NOx and achieve maximum fuel economy during combustion in the engine, notwithstanding any problems with urea distribution. In contrast, the use of urea in the U.S. will probably be more measured, because the price of urea will be closer to the price of diesel. Moreover, the problems with urea distribution and pricing are coupled with fluctuations in diesel fuel prices.